It is important to ensure that DIMMs
with appropriate number of ranks are
populated in each channel for optimal
performance. Whenever possible, it is
recommended to use dual-rank DIMMs in
the system. Dual-rank DIMMs offer
better interleaving and hence better
performance than single-rank DIMMs.

For instance, a system populated with
six 2GB dual-rank DIMMs outperforms a
system populated with six 2GB
single-rank DIMMs by 7% for
SPECjbb2005. Dual-rank DIMMs are also
better than quad-rank DIMMs because
quad-rank DIMMs will cause the memory
speed to be down-clocked.

Another important guideline is to
populate equivalent ranks per channel.
For instance, mixing one single-rank
DIMM and one dual-rank DIMM in a
channel should be avoided.

Ultimately, the effect of the number of memory ranks is specific per server/chipset. For example, on IBM's x3850X5 servers more ranks is better (see §3.8.4):

With the Xeon 7500/6500 processors in the x3850 X5, having more ranks gives better
performance. The reason is because of the addressing scheme, which can extend the pages
across ranks thereby making the pages effectively larger and therefore more page-hit cycles.

4 Answers
4

Wikipedia has a fairly good explanation of rank (link). I'd say RamCity (a vendor for Kingston memory) has a more succint explanation on ranks (link):

A memory rank is, simply put, a block
or area of data that is created using
some or all the memory chips on a
memory module.

A rank must be 64 bits of data wide;
on memory modules which support Error
Correction Code (ECC), the 64-bit wide
data area requires an 8-bit wide ECC
area for a total width of 72 bits.
Depending on how memory modules are
engineered, they can contain one, two,
or four areas of 64-bit wide data
areas (or 72-bit wide areas, where 72
bits = 64 data bits and 8 ECC bits).

The article goes on mentioning price variation:

Why do the single- and dual-rank
memory modules vary in price?

In general, single-rank memory modules
are built using x4 (“By 4”) DRAM chips
and are more expensive than dual-rank
memory modules (which are built using
x8 DRAM chips); both module types have
the same number of chips but the x4
DRAMs are more expensive than x8
DRAMs. Dual-rank memory modules may
limit future upgradeability and
capacity of servers when using PC2700
or PC2-3200 memory. This tradeoff
between memory cost and capacity is
important to consider when purchasing
memory modules for Intel
Lindenhurst-based servers.

In terms of performance, I'd refer to wikipedia:

The ranks cannot be accessed
simultaneously as they share the same
data path.

So to sum up everything, it appears that ranks have more to do with density and pricing than actual performance. Granted, I'm working off of generalized statements from a vendor and wikipedia, I don't think most people put much effort into researching ranks. All that matters (for most server admins) is that RAM have matching ranks. I don't think it's an actual specification or requirement but it helps keep some consistency and keeps memory interchangeable within a number of similar servers.

Keep in mind that most servers are upgradeable and RAM density has a large part in factor. It's best (albeit more expensive) to get the more dense RAM for servers to make room for future upgrades.

Thanks for the links - I had found the Wikipedia one but it explained a lot of 'How' without much 'Why'. So it sounds like the main implication of different ranks will be the max # of ranks supported by the chipset...
–
MikeyBSep 29 '09 at 17:24

Yes, it is partially driven by the chipset support. Ultimately for servers, rank comes down to density. Servers usually want more RAM to cram (sorry for the rhyme). But like you said.. it comes down the chipset for that quantity.
–
osij2isSep 29 '09 at 18:59

As an addendum to this; ranking does matter on some servers. Take for example the Dell R710 - if you use quad rank RAM you cannot fill more than just the A channel (from memory) and retain the full RAM speed. If you populate A and B with quad-rank RAM then the RAM speed drops to 800Mhz, and you cannot use the C channel at all. So ranking can be quite important.
–
Mark Henderson♦Dec 5 '13 at 8:35

As I know, Single Rank DIMM is more expensive than Dual Rank DIMM. In order to pack the same amount of memory on Single Rank DIMM, the manufacturers have to use bigger capacity chips to make up for it. That's why Single Rank DIMM tends to be more expensive.

I believe Rank has something to do with the number of connectors to the Chips on a DIMM. Since the number of connector is limited. To fit more memory with lower cost, they use two banks on a DIMM so they can use smaller chips to achieve same amount of memory. That's reason why we could run of banks without filling up all memory slots.